drm/i915: Lift i915_vma_parked() onto the gt

Currently even though i915_vma_parked() operates on a per-gt struct, it
is called from a global pm notify. This oddity was only because the long
term plan is to decouple the vma cache from the pm notification, but
right now the oddity stands out like a sore thumb!

Suggested-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191021183236.21790-1-chris@chris-wilson.co.uk